The present invention relates to methods and apparatus for transmitting vibrations through teeth or bone structures in and/or around a mouth.
The human ear can be generally classified into three regions; the outer ear, the
middle ear, and the inner ear. The outer ear generally comprises the external auricle and the ear canal, which is a tubular pathway through which sound reaches the middle ear. The outer ear is separated from the middle ear by the tympanic membrane (eardrum). The middle ear generally comprises three small bones, known as the ossicles, which form a mechanical conductor from the tympanic membrane to the inner ear. Finally, the inner ear includes the cochlea, which is a fluid-filled structure that contains a large number of delicate sensory hair cells that are connected to the auditory nerve.
The action of speaking uses lungs, vocal chords, reverberation in the bones of the skull, and facial muscle to generate the acoustic signal that is released out of mouth and nose. The speaker hears this sound in two ways. The first one called “air conduction hearing” is initiated by the vibration of the outer ear (eardrum) that in turn transmits the signal to the middle ear (ossicles) followed by inner ear (cochlea) generating signals in the auditory nerve which is finally decoded by the brain to interpret as sound. The second way of hearing, “bone conduction hearing,” occurs when the sound vibrations are transmitted directly from the jaw/skull to the inner ear thus by-passing the outer and middle ears. As a consequence of this bone conduction hearing effect, we are able to hear our own voice even when we plug our ear canals completely. That is because the action of speaking sets up vibration in the bones of the body, especially the skull. Although the perceived quality of sound generated by the bone conduction is not on par with the sounds from air conduction, the bone conducted signals carry information that is more than adequate to reproduce spoken information.
As noted in U.S. Patent Publication No. 2004/0202344, there are several microphones available in the market that use bone conduction and are worn externally making indirect contact with bone at places like the scalp, ear canal, mastoid bone (behind ear), throat, cheek bone, and temples. They all have to account for the loss of information due to the presence of skin between the bone and the sensor. For example, Temco voiceducer mounts in ear and on scalp, where as Radioear Bone Conduction Headset mounts on the cheek and jaw bone. Similarly, throat mounted bone conduction microphones have been developed. A microphone mounting for a person's throat includes a plate with an opening that is shaped and arranged so that it holds a microphone secured in said opening with the microphone contacting a person's throat using bone conduction. Bone conduction microphones worn in ear canal pick up the vibration signals from the external ear canal. The microphones mounted on the scalp, jaw and cheek bones pick the vibration of the skull at respective places. Although the above-referred devices have been successfully marketed, there are many drawbacks. First, since the skin is present between the sensor and the bones .the signal is attenuated and may be contaminated by noise signals. To overcome this limitation, many such devices require some form of pressure to be applied on the sensor to create a good contact between the bone and the sensor. This pressure results in discomfort for the wearer of the microphone. Furthermore, they can lead to ear infection (in case of ear microphone) and headache (in case of scalp and jaw bone microphones) for some users.
There are several intra-oral bone conduction microphones that have been reported. In one known case, the microphone is made of a magnetostrictive material that is held between the upper and lower jaw with the user applying a compressive force on the sensor. The teeth vibration is picked up by the sensor and converted to electrical signal. The whole sensor is part of a mouthpiece of a scuba diver.
US Patent Publication No. 2004/0202344discloses a tooth microphone apparatus worn in a human mouth that includes a sound transducer element in contact with at least one tooth in mouth. The transducer produces an electrical signal in response to speech and the electrical signal from the sound transducer is transmitted to an external apparatus. The sound transducer can be a MEMS accelerometer, and the MEMS accelerometer can be coupled to a signal conditioning circuit for signal conditioning. The signal conditioning circuit can be further coupled to a transmitter. The transmitter can be an RF transmitter of any type, an optical transmitter, or any other type of transmitter such as a Bluetooth device or a device that transmits into a Wi-Fi network.